1. Field of the Invention
The present invention relates to a binding device, and particular to a binding device for holding together sheets of paper or other objects and binding them into a file.
2. Description of the Related Art
FIG. 33 shows one example of a conventional binding device.
The binding device 1 includes a board 2 and a bearing plate 3 provided upright at one widthwise side of the board 2.
An operation lever 4 is pivotally attached to the bearing plate 3 so that it is rotatable in a plane that is substantially perpendicular to the board 2.
Furthermore, a pressing plate 5 is provided, which is movable towards and away from the board 2 in response to the rotation of the operation lever 4.
A spring member 6 is provided between the pressing plate 5 and the operation lever 4 for biasing the pressing plate 5 toward the board 2 when the operation lever 4 is operated. Cranks 7 are attached on both sides of this spring member 6. The cranks 7 are bridged between the bearing plate 3 and the pressing plate 5 so that the pressing plate 5 is substantially parallel to the board 2 when it is close to and separated from the board 2.
The pressing plate 5 has an inverted U-shape cross section to hold down the objects to be bound, such as paper, with both of its widthwise ends.
With this binding device 1, by operating the operation lever 4, the pressing plate 5 is displaced toward the board 2 as it moves along the bearing plate 3 so that the objects to be bound, such as paper, are held and secured therebetween.
However, one problem with this binding device 1 is that, since the pressing plate 5 moves along the bearing plate 3, it moves along one end of the objects to be bound, such as paper, which makes it difficult to set the objects to be bound, such as paper, in a binding position.
To solve this problem, another binding device shown in FIG. 34 was devised.
This binding device 8 does not include any cranks, and the pressing plate 5 is rotatably attached to a rotation axis 9a that is connected to two arms 9.
The ends of the arms 9 are rotatably attached to the bearing plate 3, and the pressing plate 5 is attached so as to be rotatable around two axes, i.e., at the attachment part on the bearing plate 3 and around the rotation axis 9a. 
With this binding device 8, by operating the operation lever 4, the pressing plate 5 rotates around the attachment parts of the arms 9 on the bearing plate 3 and around the rotation axis 9a as it is pressed toward the board 2, the pressing plate 5 being remained substantially in parallel with the bearing plate 3. Therefore, as the pressing plate 5 moves at the binding position of the objects to be bound, such as paper, the objects can be easily set at the binding position (see Japanese Patent Laid-Open Publication No. Hei 7-246794 (Patent Document 1)).
FIG. 35 shows another binding device 12 that has a plate-like holding plate 10 rotatably supported at pivotal support parts 11 on the board 2, so that objects to be bound, such as paper, are bound on one side of the pivotal support parts 11.
With this binding device 12, a spring member 6 is provided to bias the other side of the pivotal support parts 11 of the holding plate 10 so as to hold together the objects to be bound, such as paper, at one side of the pivotal support parts 11 of the holding plate 10 (see Japanese Utility Model Laid-Open Publication No. Sho 49-76714 (Patent Document 2)).
However, with the binding devices shown in FIG. 33 and FIG. 34, the objects to be bound, such as paper, are pressed with both widthwise ends of the pressing plate that has an inverted U-shape cross section, which means that the force exerted by the spring member is dispersed, causing the bound objects, such as paper, to readily come off.
The problem with the binding device shown in FIG. 35 is that since it has a structure in which the bound objects, such as paper, are held on one side of the pivotal support parts of the holding plate while the other side of the pivotal support parts is biased with the spring member, considering the size of the entire binding device that is attached to a file, the length from the pivotal support parts to the binding side is longer than that to the spring member side. Accordingly, the force applied at the binding side of the holding plate is less than the biasing force applied by the spring member, which makes it likely that the bound objects, such as paper, will readily come off.